This chapter delineats the rationale of combining the membrane and bioreactor technologies and systematically introduces the membrane biological reactors that are the focus of this book. This is followed by a brief account of the history of the various applications of membranes in conjunction with bioreactors and how the initial formats led to the development of the present day commercialized ones. The relative advantages of membrane biological reactors over the conventional biological processes are presented. The current status of the MBR market is described and the drivers propelling its growth along with the constraints are highlighted. The trends in world-wide MBR research is succinctly presented in order to assess whether academic research has so far been able to maintain close relationship with the specific requirements in the industry.